Many types of communication systems use a power amplifier as part of the front end of a transmitter for transmitting RF signals. In some systems, a low noise amplifier (LNA) at the front end of a receiver may be used for receiving RF signals. However, a power amplifier has non-linear distorting characteristics that can cause distortion of the information signal being amplified. The non-linear distorting characteristics of the power amplifier can impact the instantaneous amplitude and phase of the signal significantly. Furthermore, non-linearity within the RF components can introduce distortion in the signal and reduced SNR or poorer performance at the receiver. In wireless systems, non-linearity in the power amplifier at transmission can introduce intermodulation distortion which can result in spectral emissions in the adjacent channels. Typically, tight specifications in terms of spectral masks are provided by Standards Committees and/or Government bodies, which put a maximum limit to such spurious, out-of-band emissions for compliant transceivers. Therefore, it is desirable to provide a linear signal out of the power amplifier.
One type of method to linearize the power amplifier output signal is to “pre-correct” the signal being input to the amplifier, also known as pre-distortion. There are many known techniques are used to pre-correct an information signal in order to linearize the output of the amplifier. One of these techniques involves amplitude correction which produces a linear piece-wise pre-correction function which is correlated to the non-linear characteristics of the amplifier. The result is a piece-wise correction curve which approximates the ideal correction. The correction is then added to the information signal.
Pre-distortion has been almost exclusively in the baseband domain. The typical approach has been to apply a pre-distortion function at digital baseband. A relatively recent but commonly used approach has been to store the pre-distortion function as a look-up table which stores the gain and phase values as a function of the input signal envelope. The input signal is compared with the feedback signal from the power amplifier and certain metrics, such as ratio of in-band to out-of-band emission power and/or their correlation are used to adaptively map the look-up table with respect to the envelope of the input signal. Thus, the data path is simply a complex multiplication at digital baseband; the adapt-path (or feedback path) consists of a look-up table indexed by the input signal envelope and the adaptation metric is the ratio of in-band to out-of-band emission or the correlation between the feedback and the input signals. These operations are carried out at digital baseband.
However, increasing the linearity of the power amplifiers can reduce the power efficiency of the amplifiers, make them more voluminous, require more cooling equipment, and substantially increase the cost and form-factor of the transceivers. To improve the linearity of the transmission without resorting to a higher end and more expensive power amplifier, one method that has been carried out in the prior art is to perform pre-distortion on the transmitted signal before the signal is input into the power amplifier such that the pre-distortion equalizes in some sense the non-linear post-distortion of the amplifier. The pre-distortion may be carried out within an integrated circuit; however, a significant challenge has always been to be able to adaptively obtain the pre-distortion transfer function so as to be optimal in some metric. It is also desirable to have a tracking mechanism with the pre-distortion so that different temperature and aging effects are also compensated for.
Accordingly, it would be desirable to have systems and methods for performing pre-distortion in power amplifiers that overcome the disadvantages of the prior art as discussed.